Introduction
Crowding is a common practice in salmon aquaculture during operations such as sea lice treatments and harvesting. Sea lice , particularly Lepeophtheirus salmonis, remain one of the most significant parasitic challenges in Atlantic salmon farm ing. A growing concern is that during crowding events, mobile lice detach from their hosts and spread into the surrounding environment where they may pose a reinfestation risk for both farmed and wild fish. Despite these concerns, the extent to which lice detach and are lost during these events is still not well understood. The aim for this project was therefore to quanti fy lice detachment , determine the mesh size needed to capture all lice stages , and assess the feasibility of using a fine-meshed net to collect detached lice during crowding.
Methods
Replicate crowding trials were conducted in small research cages, using either a standard coarse -mesh crowding net or a fine-meshed crowding net intended to retain detached lice. The trials varied in crowding duration (10 min – 2 h), fish size and number, a s well as louse density . Detached lice that passed through the crowding nets were captured using a plankton net, allowing direct quantification of lice that were ‘lost’ during the crowding process. Additionally, a benchtop physical model was made simulating the retention of detached lice using various mesh types and sizes (0.8–2.0 mm) as well as precise measurements of each louse stage size metrics .
Results and discussion
Mobile lice that detached during crowding varied from 2 to 38%, with higher detachment rate s during longer crowding duration (up to 2 h) and with larger fish. Smaller pre-adult lice detached more frequently than adult lice . Laboratory studies demonstrated that a nominal mesh size of 0.8 mm effectively captured all lice stages , 1.6 mm retained most pre-adults and adult males, while adult females required a 2.0 mm mesh for successful collection. The maximum mesh opening size (diagonally ) corresponded to the cephalothorax width of the louse stage captured. In cage trials, the fine-meshed crowding net successfully retained 75% of adult female lice and improved fish welfare. Overall, these findings show that significant lice loss can occur during handling of salmon and that collecting lice during crowding operations could be an effective management strategy to limit their release into the environment and thereby lower the reinfestation risks for both farmed and wild fish.